Assays can be performed to determine the presence and/or the amount of one or more compounds of interest (e.g., analytes). Some assays can determine the presence of multiple analytes. Such assays can be performed using an array of multiple different compounds (e.g., DNA fragments) positioned at specific locations on a surface. The compound at each position interacts with different respective analyte. In use, the array is contacted with a sample containing one or more analytes. The interaction between the compound at each position and the analytes can be detected (e.g., via an optical label) and is indicative of the presence of a particular analyte.